This invention relates generally to optical devices for fiber-optic systems. More particularly, it relates to a new type of optical isolators with arbitrarily low polarization mode dispersion.
Fiber-optic networks are emerging as the backbone of modern communications systems. An important building block in a fiber-optic network is an optical isolator. This is a non-reciprocal device that permits light signals to travel in one direction and blocks light signals propagating the opposite direction. Optical isolators are generally employed in fiber-optic networks near the output ports of laser diodes which provide light signal sources in optical fibers. Because laser diodes are particularly sensitive to signal reflections, which cause the wavelength of the diode output signal to drift and adversely affect the power of the output signal, optical isolators are implemented to block such reflections and to help xe2x80x9clockxe2x80x9d the output of the laser diodes.
The optical isolators in the prior art typically comprise a plurality of birefrigent crystals or polarizers, one or more Faraday rotators, and a pair of microlenses such as gradient-index (GRIN) lenses to couple the optical signal from one fiber to another. The principles of their operation are as follows. In a forward direction, a first birefrigent polarizer splits an input light signal into ordinary and extraordinary rays. After passing through a 45xc2x0 Faraday rotator, the ordinary and extraordinary rays enter a second birefrigent polarizer, whose optical axis is at 45xc2x0 relative to the optical axis of the first polarizer. Thus, the ordinary and extraordinary rays propagate through the second polarizer as ordinary and extraordinary rays, respectively. The net result is that after traveling from the first polarizer through the second polarizer, the two rays become parallel, but are displaced from each other. An output GRIN lens then focuses them at a point on the end of an output fiber. In the backward direction from the second polarizer to the first polarizer, the second polarizer splits an incoming light into ordinary and extraordinary rays. Upon passing through the 45xc2x0 Faraday rotator, the polarizations of the light in two rays are rotated 45xc2x0 in the same direction. This rotation is non-reciprocal with the rotation of light in the forward direction, so that the ordinary ray from the second polarizer becomes the extraordinary ray in the first polarizer and the extraordinary ray from the second polarizer becomes the ordinary ray in the first polarizer. That is, the ordinary and extraordinary rays have exchanged places. Because of this exchange, the light does not leave the first polarizer in parallel rays. This non-parallel light is then focused by an input GRIN lens at points that are not located at the end of the input fiber. Hence the light traveling in the backward direction is not passed back into the input fiber, that is, it is effectively xe2x80x9cblockedxe2x80x9d.
Though the optical isolator as described above has achieved the effect of blocking backreflected and other light traveling in the backward direction, it also inadvertently produces an undesirable effect. Because a birefrigent crystal has two different indices of refraction, ordinary and extraordinary rays traversing through the optical isolator often travel two different optical path lengths. Thus, even though the signal is recombined later by the output GRIN lens, the signal may have one mode of polarization ahead of, or behind, the other. Consequently, there is a time delay between the two polarization modes and the signal becomes dispersed. This time delay is known in the art as Polarization Mode Dispersion (PMD). As the signal travels through an optical fiber over a long distance, the dispersion effect worsens. Moreover, the presence of numerous optical isolators employed in a fiber-optic network further compounds the problem.
A great deal of ingenuity has been applied in the art to minimize PMD in optical isolators. The most common practice in combating PMD thus far has been to insert a compensation plate of birefrigent material in the optical isolator as described above, as shown in FIG. 1. A Faraday rotator 10 is disposed between a first polarizer 11 and a second polarizer 12 along an illustrative optical path 13. The first polarizer 11 and the second polarizer 12 are arranged such that any light traveling from the second polarizer 12 to the first polarizer 11 will be effectively blocked, as explained above. A compensation plate 14, which may be inserted on either side of the Faraday isolator 10 in the optical path 13, is disposed to the right of the second polarizer 12 as a way of illustration. The compensation plate 14 is arranged to have its fast axis, the axis of the birefrigent material having smaller index of refraction, perpendicular to the fast axis of the polarizer on the same side of the rotator, i.e., the second polarizer 12 in this case. Moreover, its thickness and indices of refraction are so chosen to equalize the difference in optical path lengths traveled by ordinary and extraordinary rays of the optical signal through the optical isolator, thereby reducing PMD.
In order to achieve a perfect compensation in the optical isolator, however, the thickness of the compensation plate must be exact with little tolerance. This requires high-precision manufacturing of birefrigent crystals, which is costly and time consuming. As fiber-optic systems rapidly grow in communications networks; there is an urgent need for optical isolators with ever-lower PMD that can be produced easily and inexpensively.
Accordingly it is a principal object of the present invention to provide a novel tunable module that can be employed in various optical devices. It is another object of the present invention to incorporate such a tunable module in an optical isolator, such that PMD can be optimized in a controlled manner. It is a further object of the present invention to provide methods for incorporating the tunable module in optical devices and for modifying the characteristics of optical signals. It is an additional object of the present invention to provide methods for making optical isolators with arbitrarily low PMD.
The primary advantage of the present invention is that it provides optical devices with an active feedback mechanism, such that PMD, or other optical characteristics, can be optimized in a controlled manner. More specifically, by incorporating a tunable module in an optical isolator, PMD can be reduced to an arbitrarily low-level without having to precisely manufacture birefrigent crystals in the isolator. Further advantages of the present invention are evident in the simple and novel design of the tunable module and the ease with incorporating it in optical devices.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
This invention provides a novel tunable module for optical devices; and the methods for incorporating it in optical devices, particularly in optical isolators, to optimize PMD and other optical characteristics.
The tunable module of the present invention comprises one or more optical elements that are movable relative to each other, such that the optical path lengths of an optical signal traversing through the module can be varied.
The change in the optical path lengths can be achieved by employing optical elements that have uniform indices of refraction, but are tapered in thickness along the path of the optical signal; or by using optical elements that are uniform in thickness along the path of the optical signal, but bearing a gradient of indices of refraction in a direction perpendicular to the path of the optical signal; or a combination of both.
The present invention provides an optical isolator incorporating the tunable module described above. The optical isolator comprises a first birefrigent polarizer, a Faraday rotator, a second birefrigent polarizer, and a tunable module disposed along the path of an optical signal. The two birefrigent polarizers are arranged on opposite sides of the Faraday rotator, such that any light traversing from the second polarizer to the first polarizer will not be passed back into the input fiber, i.e., effectively blocked. The tunable module can be disposed on either side of the Faraday rotator. The tunable module further comprises one or more birefrigent elements.
PMD of the optical signal is reduced by arranging the fast optical axis of the tunable module to be perpendicular to the fast optical axis of the polarizer on the same side of the Faraday rotator. In the present invention, PMD is further minimized by adopting an active feedback mechanism. This involves adjusting the positions of the movable elements in the tunable module so to vary the optical path lengths of the optical signal, while monitoring the PMD of the optical signal, until the PMD is minimized. The positions of the movable elements are then secured by means of an adhesive agent, such as an epoxy, or other suitable coupling. In the optical isolator thus constructed, the optical path lengths of light rays in different polarization modes are optimally equalized.
In the above optical isolator the tunable module and polarizers can be formed from various birefrigent materials, such as calcite, lithium niobate, rutile, and YVO4 based crystals. They are typically made from the same type of birefrigent material for the ease of manufacturing, though they can also comprise different birefrigent materials.
The tunable module of the present invention can be incorporated in other optical devices in a manner similar to that is describe above, to provide a tunability and selectivity in a characteristic of an optical signal.
The present invention further provides an optical isolator comprising a Faraday rotator, first and second birefrigent polarizers arranged on opposite sides of the Faraday rotator, and a birefrigent compensation plate disposed along the path of an optical signal. The two birefrigent polarizers are arranged such that any light traversing from the second polarizer to the first polarizer will be blocked. Moreover, at least one of the birefrigent polarizers is a movable polarizer with respect to the other such that the optical path lengths of the optical signal traversing through the movable polarizer can be varied. The compensation plate can be disposed on either side of the Faraday rotator.
The compensation plate and the polarizers can be formed from various birefrigent materials, such as calcite, lithium niobate, rutile, and YVO4 based crystals. They are typically made from the same type of birefrigent material for the ease of manufacturing, though they can also comprise different birefrigent materials.
PMD is reduced by arranging the fast optical axis of the compensation plate to be perpendicular to the fast optical axis of the polarizer on the same side of the Faraday rotator. PMD is further minimized by employing an active feedback mechanism. This involves adjusting the positions of the movable polarizer so to vary the path lengths of the optical signal, while monitoring the PMD of the optical signal, until the PMD is minimized. The position of the movable polarizer may then be secured by means of an adhesive agent, such as an epoxy, or other suitable coupling. In the optical isolator thus constructed, the optical path lengths of light rays in different polarization modes are optimally equalized.
The novel features of this invention will be best understood from the following drawings and detailed description.